More highlight drawing fixes

This fixes a couple more highlight drawing problems uncovered in issue #3326 

1. The highlight box was too small. The last checkin changed to `box-size: border-box`, but the width and height calculations were adjusting for the border. Those adjustments are removed.
2. Set `padding: 0` and `margin: 0`. This way if the app has a `div` selector with paddings and/or margins set, the highlight doesn't pick them up.